When zooming in or out, if a tile is not yet cached, Maps now also
checks the lower and higher zoom levels for cached tiles and composites
a preview tile that is shown until the actual tile is loaded in.
Previously we would bombard RequestServer with as many requests as it
would take, but when frantically zooming in and out, we would run out of
fd's causing the Maps application to hang.
This implements a tile download queue and limits the amount of parallel
downloads to 8. This fixes the hangs accompanied by this debug console
message:
Peer endpoint error: recvfd: Too many open files (errno=24)
It does not fix other sporadic hangs caused by requests that are never
finished, however.
